It is generally believed that soil is composed of solid phase (soil particles), liquid phase (pore water) and gas phase (gas contained in soil). When the interspace of soil particles are completely filled with liquid phase, the soil is defined as saturated soil. Otherwise, when the interspace in soil is filled with the water and air, that is, when the saturation degree is lower than 100%, the soil is defined as unsaturated soil.
The geotechnical engineering usually involves the problems about engineering properties of unsaturated soil. Study on the engineering properties of unsaturated soil should start with the soil water characteristics of unsaturated soil. No matter in the application field or research field, providing accurate soil water characteristics of unsaturated soil is of great importance for the geotechnical engineering technology. The soil water characteristic curve of unsaturated soil is used to reflect the relationship between the matrix suction and the volumetric water content, which has important significance for determining the shear strength, volume change and permeability coefficient of unsaturated soil. The soil water characteristic curve has significant hysteresis phenomenon, i.e., the dehydration curve shall be higher than the soaking curve, and both form a hysteresis loop. This means that under the same net pressure and matrix suction, soil may have different saturation degrees, and may show different shear strengths and permeability coefficients. Soil hysteresis problems extensively exist in the engineering practice. For example, affected by the water level change in the reservoir area, side slope will be bound to experience repeated dehydration-soaking-dehydration process. Therefore, accurately testing the hysteresis phenomenon of unsaturated soil has very important significance for study on the unsaturated soil theory and the engineering design.
The air entry value (AEV) of unsaturated soil means the matrix suction required to produce unsaturation in the maximum pores within the soil mass, and is an important parameter for dividing saturated state or unsaturated state of the soil mass. Precision of AEV has great influence on the dehumidification scan line with the starting point in the high saturation area, and also plays a crucial role in capillary block impervious layer design of the engineering application. Therefore, accurate determination of the AEV of materials becomes a very important work in the hydraulic properties test of unsaturated soil.
Pressure plate apparatus is one of the basic devices to determine the soil water characteristic curve of unsaturated soil. The commonly used test device includes: Fredlund SWCC pressure plate apparatus, Geoexpert pressure plate apparatus and a soil water characteristic curve tester provided in Chinese utility model patent No. 201120271620.0. According to these test methods, because of the migration effect in the discharge water test, bubbles are generated below the ceramic plate, and are collected in the drain pipe. The volume of the bubbles is easy to be mistaken for the discharge water amount, thereby affecting the accuracy of the water discharge test. At present, bubbles are usually discharged by means of repeated washing to solve this problem, which not only causes more cumbersome operation steps, but also increases the difficulty of the test work. Therefore, developing a test device capable of automatically discharging bubbles from the base has important significance for accurate test of the soil water characteristic curve.
Besides, at present, in the process of air entry value determination of unsaturated soil with the pressure plate apparatus based on the axis parallel translation technique, it is generally assumed that the water pressure below the soil mass is zero, that is, the applied pressure is the matrix suction in soil. However, the drainage water measuring tube of the above apparatus is usually fixed on the bench of the apparatus, and the initial liquid level in the measuring tube is generally 20˜30 cm higher than the soil sample. Actual water pressure of 2˜3 KPa will be produced below soil, and will be improved with the rise of the water level in the drainage water measuring tube. This changing water pressure often existing in soil has very significant influence on the test precision of air entry value of unsaturated soil, as well as the hysteresis curve test. Specifically, the above test apparatus has three defects as follows:
1. At low matrix suction, due to reduced air pressure and water pressure effect in the water inlet in the soaking experiment, excess water will be collected on the ceramic plate, and there will be the phenomenon such as macerated soil samples, resulting in water inflow test error.2. It is impossible to accurately test the air entry value. The air entry value of the soil sample is generally 5˜20 KPa. Therefore, 2˜3 KPa error caused by 20˜30 cm water head has very significant influence on the air entry value test.3. It is impossible terminate the sampling test in the process of the soil water characteristic hysteresis curve test of unsaturated soil. Therefore, the initial soil samples are required to be in saturated state. The soil samples are traditionally saturated usually using outdoor pressure head or back pressure, and then tested in a pressure chamber. But any method has the problem of disturbing soil samples in handling, and saturated original soil samples tend to be slightly liquefied due to shock, so that some of the original interspaces turn smaller, thereby changing the structural style of the undisturbed soil, affecting precise air entry value test of the soil mass, and changing the shape of the soil water characteristic curve of undisturbed soil.
Therefore, developing a device that allows soil samples to be in a state of zero water pressure gradient in the process of dehydration and soaking, and is capable of vacuum saturation of the test samples directly in the pressure chamber has important significance for accurate determination of hysteresis problems of soil water characteristic curve of unsaturated soil.